Lawrence Lessig, who argued before the Supremes in Eldred vs. Ashcroft, gives his analysis of how the Court reacted — and, in so doing, describes the argument behind Eldred’s case very nicely indeed.
The government then helped us immensely by simply confirming what we had said: under their theory of the case, there was no constitutional limit on Congress’s power to extend terms; it was always a matter of Congress’s discretion. Congress could perpetually extend existing terms; it could even extend a copyright to works within the public domain.
The Court clearly did not like this answer. They had bought the idea that the Constitution intended there to be a limit; the government’s interpretation meant that this was a limit that was solely a matter of legislative grace. (Compare: “Under our written Constitution the limitation of congressional authority is not solely a matter of legislative grace.”) They were not comfortable with the idea that they would simply say that though the constitution expressly limits Congress’s power, it is Congress that gets to say what that limit is.
Y’know, once upon a time, I seriously considered becoming a lawyer? I love this stuff.